1. Field of the Invention
The invention relates in general to calibration of coordinate measuring machines. In particular, it relates to calibration performed by optical measurements using an optical flat.
2. Description of the Related Art
A coordinate measuring machine (CMM) is a device for measuring the physical geometrical characteristics of an object, typically for quality control purposes in manufacturing and assembly processes. The typical CMM is composed of three axes (X, Y, Z), orthogonal to each other in a typical three-dimensional coordinate system. The first horizontal axis (typically the Y axis) is defined by a bridge or gantry supported by two vertical legs coupled to a stationary support table. The second axis, X, is defined by the horizontal motion of the bridge along the support table in the direction normal the first axis (thereby defining an XY plane). The third, Z axis is defined by the vertical motion of a quill or spindle attached to the bridge. A probe is attached to the quill for contact or optical measurements of a part based on a scale system that indicates the location of the probe along each axis. In operation the machine reads the input from the probe as it traces the part at various points and the X,Y,Z coordinates of these points are used to determine size and shape with micrometer precision. The typical CMM takes readings in six degrees of freedom and displays these readings in mathematical form. For the purposes of this disclosure the mechanism providing the three dimensional motion of the probe in relation to the sample part (or vice versa) is defined as the carriage mechanism of the CMM.
Of course, the motion of the probe along each axis is not perfect and six main errors are associated with each straight-line motion. As illustrated in FIG. 1 with reference to the Y axis, for example, these errors are identified in the art as the linear or scale positioning error (indicating that the probe does not move the measured distance along the axis), the two straightness errors (the probe moves up/down or left/right with respect to axis), the pitch error (angular front/back motion), the roll error (angular left/right motion), and the yaw error (rotation around the Z axis). In addition, three so-called squareness errors relate to the alignment among the three axes. Thus, a total of 21 errors may be introduced by the mechanical translation of the probe during the measurement of an object. Therefore, the ability to estimate these errors and correct the results of measurements through calibration of the coordinate measuring machine or otherwise is essential in the operation of CMMs.
With the advent of laser interferometry, it has become possible to calibrate coordinate measuring machines to the degree required to correct nano-scale errors. For example, laser interferometers available commercially from manufacturers such as Agilent and Renishaw (Products No. ML10 and No. XL-80) are equipped to measure pitch, yaw, straightness, and linear errors (roll errors cannot be calibrated with laser interferometry). Squareness errors can also be calculated from data obtained through laser interferometry, but typically they are obtained from so-called “ball bar” or “length bar” measurements, which are simpler, less expensive and more accurate, and thus preferred. In all cases, optimal error correction of CMMs is achieved by providing a total of 21 error parameters (six for each coordinate axis plus three for squareness) that are then used to calibrate the machine and correct the measurement results.
The operation of laser interferometers in the context of CMM error calibration and correction and the attendant mathematical formulation of the results of error measurements are well known in the art. Therefore, they are not described here. For reference, see U.S. Pat. No. 4,819,195 and U.S. Pat. No. 4,939,678, and G. Zhang et al., Error Compensation of Coordinate Measuring Machines, CIRP Annals, Volume 34, Issue 1, pp. 445-448 (1985), herein incorporated by reference. Because of the precision afforded by such optical measurements, the use of laser interferometry has become the conventional approach where high precision is required. However, such interferometers are relatively large pieces of equipment that are complicated to use for CMM calibration and are not suited for measuring errors of smaller CMMs having a typical working space in the order of centimeters. This invention is directed at a novel approach for calibration and error correction of coordinate measuring machines based on the use of an optical flat in combination with an optical topography probe for 3-D measurements, such as an interferometric microscope objective.